(a) Field of the Invention
The present invention relates to a resonant converter, and more particularly, it relates to a converter that performs a zero voltage switching operation and a driving method thereof.
(b) Description of the Related Art
A driving voltage and a driving current are input to a resonance network of a resonant converter. The driving voltage and the driving current are defined as a voltage and a current that are input to the resonance network of the resonant converter. The driving current is preferred to have a waveform with a phase that is slow compared to that of the driving voltage. As a switch that controls operation of the converter, a transistor element is typically used. When the transistor is being turned on, the overlap area of transistor current and voltage makes switching loss. Thus, when the transistor is being turned on, it is preferred that a voltage difference between drain and source electrodes is small in order to minimize a switching loss. When the driving current has a lagging phase compared to the driving voltage, a current flowing through a body diode of the transistor while the transistor is being turned on is generated. Then, a voltage difference between the drain and source electrodes of the transistor during turn-on transition is reduced, thereby reducing the switching loss.
An inductive region and a capacitive region can be divided in accordance with an impedance characteristic of the resonant network of resonant converter. If a switching frequency is faster than a resonant frequency of the resonant network, the resonant converter operates in the inductive region. If the resonant frequency is faster than the switching frequency, the resonant converter operates in the capacitive region. This is according to frequency dependent characteristics of the impedance of resonant network. When the network has inductive impedance, the impedance increases in proportion to frequency while the impedance changes inversely proportional to the frequency when the network has capacitive impedance. While the resonant converter operates in the inductive region, the driving current has a lagging phase compared to the driving voltage. Because the operation in the inductive region minimizes switching loss, the resonant converter is always designed to operate in inductive region. When the resonant converter operates in inductive region, the input power increases as switching frequency decreases since the input impedance decreases as the frequency decreases.
A switching frequency of the converter may vary according to a load connected to an output end of the resonant converter. When the output load increases, the controller decreases the switching frequency so as to decrease the input impedance and therefore to increase input power. In further detail, when the converter is overloaded, the switching frequency is decreased in order to obtain the maximum gain. When the switching frequency is decreased to be smaller than the resonance frequency, the converter operates in the capacitive region. When the resonant converter operates in the capacitive region, a reverse recovery current is generated due to a body diode of the transistor that is a switching element in the switching operation. When a backward voltage is applied while a current is flowing forward to the diode, the current gradually becomes zero after the current flows backward rather than immediately becoming zero. This current is referred to a reverse recovery current. Due to this current, serious switching noise and switching loss are generated. In addition, the resonant converter may operate in the capacitive region in the case in which an output end is short circuited.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.